What to do if my iPhone XR is stolen and "Find My iPhone" is off?

If Find My was not set up prior to the iPhone being stolen, then no, Apple cannot help. There will be no way anyone can track the device.

Sorry, but it's probably gone forever, unless the police get really lucky.

Regards.

There is no way for you to track the phone. Hopefully it was locked with a passcode the thieves do not know.

You will likely never see it again, but you can ask your carrier to blacklist it, so that no carrier who honors the blacklist will provide cellular voice, text message, or data service to it ever again.

What to do if my iPhone XR is stolen and "Find My iPhone" is off?

Even if Find My was enabled, if a phone is stolen the chances of ever seeing it again are very small. With Find My not enabled, there is no chance that the phone will ever be located.

Hopefully, you did have a passcode enabled on the phone.

If your iPhone or iPad is lost or stolen - Apple Support

Apple does not know the phone's iCloud, email or phone number.

Apple does not get involved in stolen devices.